This invention relates to steam turbines, and more particularly, to an apparatus and method for improved moisture extraction from low pressure steam turbines operating at low load.
It is well-known that water droplets entrained in steam flow through a steam turbine system can cause serious erosion damage to system hardware. The erosion problem has been thoroughly discussed in a number of publications. For instance, U.S. Pat. No. 4,527,396 assigned to Westinghouse Electric Corporation discloses a moisture preseparator for removing erosion-causing entrained liquid from effluent traveling through a steam turbine exhaust system.
Accordingly, it has long been an object of steam turbine design to reduce erosion damage throughout the system by removing moisture content from the flow of live steam at a plurality of points along the route from turbine inlet to exhaust. One of these locations in at least one type of low pressure steam turbine is just upstream of the last rotating blade of the turbine, where an annular moisture extraction slot has been incorporated into the turbine casing. Moisture entering this extraction slot drains to a condenser. Steam entrained water droplets are propelled by the turbine blading to the casing where the droplets are suctioned to the condenser by virtue of a pressure differential.
Erosion damage studies performed on low pressure steam turbines at several power plant installations have resulted in data that indicate that at low loads such as, for example, less than about twenty percent, there is an insufficient pressure drop from the nozzle inlet of the last rotating blade tip to the condenser, to create sufficient suction to fully drain the water that collects in the annular collection slot. Since this water tends to dribble back into the blade path in the form of large droplets if it is not exhausted, the collected moisture may increase erosion of the last stage turbine blading. Additionally, condensation in the steam flow reduces the efficiency of the turbine.
At low loads, the water droplets tend to be larger and not entrained well by the steam. Larger droplets with their increased mass have been found to increase the erosion problem. A substantial portion of first-year erosion of turbines in nuclear installations is believed due to many hours of low-load operation, i.e., at loads below twenty percent, mandated by regulations applicable to nuclear reactor operations.
It is therefore an object of this invention to reduce low-load erosion damage in a steam turbine by improving moisture extraction adjacent a last rotating blade row in the turbine.